This invention relates generally to a heat-transfer base plate such as a heat spreader and a heat sink of a semiconductor device etc., and more particularly to a composite material for an electronic part, in which heat-dissipativity in a direction of the thickness of the plate is improved. The invention also relates to a semiconductor device and a method of producing such a composite material.
Generally, a ceramic package, called a PGA (Pin Grid Array), is used in a CPU (central processing unit) of a large-scale computer, a work station, a personal computer (PC) and so on, and heat, generated from a silicon chip, is dissipated through a heat-transfer base plate (heat spreader) provided between the silicon chip and aluminum heat sink fins. With a high-speed, high-power consumption design of recently-developed LSIs, it has now become quite important to effectively dissipate heat generated from a silicon chip, and particularly in an LSI and the like for a microcomputer and a logic ASIC (Application Specific IC), a heat spreader is disposed in contact with a silicon chip, thereby promoting the dissipation of heat.
For example, a BGA (Ball Grid Array) package, shown in FIG. 4, comprises a heat spreader 11, a silicon chip 8, Cu wiring 9, a polyimide film 10 for insulating purposes, and solder balls 12 serving as terminals. In this structure, the heat spreader 11 is held in contact with the silicon chip 8, and it is important that the heat spreader should have sufficient heat-dissipativity to relieve heat generated from the silicon chip 8 and that the heat spreader 11 should match in thermal expansion coefficient with the silicon chip 8. It is expected that there will be an increasing demand for this new type of package.
Since this heat spreader is in contact with the silicon chip, the heat spreader must match in thermal expansion coefficient with the silicon chip, and generally it is preferred that its thermal expansion coefficient be 4.about.11.times.10.sup.-6 /.degree. C.
To meet such characteristics requirements, a plate (having a thickness of 0.5 to 1 mm, a width of 30 mm and a length of 30 mm), made of Cu--W or Mo, has heretofore been used to provide a heat spreader. However, these materials are expensive, and also have a large specific gravity, and therefore the weight of the package is inevitably heavy, and this is a great disadvantage from the viewpoint of down-sizing which is a recent trend in LSIs.
Apart from the above-mentioned BGA-type LSI, in an LSI package of the conventional type using a lead frame, there has been used a method in which the lead frame itself is made of copper or a copper alloy having good heat dissipativity. In this case, the lead frame is higher in thermal expansion coefficient than a silicon chip, and therefore an internal stress, developing in the interface between the silicon chip and the lead frame, offers a problem, and there is a possibility that a crack develops in the silicon chip during the process or when this package is in use. In order to provide a material overcoming this problem, the inventors of the present invention et al have earlier filed a Japanese Laid-Open Patent Publication No. 8-232049 directed to a composite material for an electronic part and a method of producing the same, in which powder, composed mainly of copper or a copper alloy, is sintered to form a sintered layer on at least one side (or face) of a thin plate of a Fe--Ni alloy having a low thermal expansion coefficient.
However, in those packages (e.g. a BGA package) not using a lead frame, even if copper and a Fe--Ni alloy are merely combined together to form a multi-layer structure, it can not be applied to a heat spreader since thermal conductivity is poor in a direction of the thickness of the plate (that is, in the direction of lamination). Therefore, it has now been required to provide a heat spreader which is inexpensive, small-sized, thin and lightweight, instead of the Cu--W plate and the Mo plate. With respect to those packages not using a lead frame, the above PGA and BGA packages and a CSP (Chip Size package) have been put into practical use, and it is expected that there will be an increasing demand for these packages.
Japanese Patent Examined Publication No. 7-80272 discloses a five-layer composite material for a heat spreader in which a low thermal expansion metal plate having a plurality of through holes extending in a direction of a thickness thereof, is integrally bonded by rolling to each side (or face) of a plate of copper or a copper alloy, to provide a three-layer structure, so that the copper or the copper alloy is extended in the through holes, and a sheet of copper or a copper alloy is further bonded by rolling onto each side of the above three-layer structure so that five-layer composite material may be formed.
In this method, because of the copper or the copper alloy filled in the through holes, the high thermal conductivity in the laminated direction, which would not be obtained with a simple laminated structure, can be secured, and this is effective for the heat spreader.
However, a study by the inventors of the present invention et al have found problems that the composite material, disclosed in Japanese Patent Examined Publication No. 7-80272, is the five-layer structure at most, and that the composite material is liable to be warped through the heating of the end surfaces since the layers are different in thermal expansion characteristics.
In the production method disclosed in Japanese Patent Examined Publication No. 7-80272, the cold rolling is used, and even if diffusion annealing is used after the layers are laminated together, a diffusion layer, formed between the adjacent layers, is thin, so that non-bonded portions may be partially formed, and therefore this composite material to be joined to form the semiconductor device is not entirely satisfactory in reliability.